How can I copy an instance of a class in C#

Question

Is there a way to copy an object in C#? Something like:

var dupe = MyClass(original);

I want them to be equal such that all data members are identical, but not share the same memory location.

Lukin

One possibility is to use reflection to create a generic method to do this for you: pmichaels.net/2016/03/04/… – Autolithography

Answer 1

You are probably talking about a deep copy (deep copy vs shallow copy)?

You either have to:

1. implement (hard code) a method of your own,
2. try to implement (or find) an implementation that uses Reflection or Emit to do it dynamically (explained here),
3. use serialization and deserialization to create a deep copy, if the object is marked with a [Serializable] attribute.

public static T DeepCopy<T>(T other)
{
    using (MemoryStream ms = new MemoryStream())
    {
        BinaryFormatter formatter = new BinaryFormatter();
        formatter.Serialize(ms, other);
        ms.Position = 0;
        return (T)formatter.Deserialize(ms);
    }
}

To get a shallow copy, you can use the Object.MemberwiseClone() method, but it is a protected method, which means you can only use it from inside the class.

With all the deep copy methods, it is important to consider any references to other objects, or circular references which may result in creating a deeper copy than what you wanted.

⚠️ Security warning: Please read about the danger of using BinaryFormatter which may include remote code execution... You can instead use the preferred alternatives listed in the provided link.

Lappet

Serialization is a nice idea :-). – Cupid

@Martin: Yes, that's the easiest way, although to use native .NET serialization class must be marked as [Serializable]. Otherwise you have to use some other implementations. – Lappet

For some reason the above code snippet is missing <T>. For example I should be seeing: public static T DeepCopy<T>(T other) – Masse

Answer 2

Not all classes have this functionality. Probably, if a class does, it provides a Clone method. To help implement that method for your own classes there's a MemberwiseClone protected method defined in System.Object that makes a shallow copy of the current instance (i.e. fields are copied; if they are reference types, the reference will point to the original location).

Vickers

Answer 3

If your class has just got properties, you could do something like this:

SubCentreMessage actual;
actual = target.FindSubCentreFullDetails(120); //for Albany
SubCentreMessage s = new SubCentreMessage();

//initialising s with the same values as 
foreach (var property in actual.GetType().GetProperties())
{
    PropertyInfo propertyS = s.GetType().GetProperty(property.Name);
    var value = property.GetValue(actual, null);
    propertyS.SetValue(s, property.GetValue(actual, null), null);
}

If you have fields and methods, I am sure you can recreate them in new class using reflections. Hope this helps

Printer

Answer 4

Yes and no. This is an area where you need to be a bit careful because there are some traps (and then again, it's not difficult).

First of all, if you poke around in the Base Class Library (BCL) a bit, you may discover the ICloneable interface. Not all types implement this interface, but those that do have a Clone method that will return a new instance of the same type with (presumably) the same values.

However, herein lies a trap: The ICloneable interface does not sufficiently specify whether a deep clone or a shallow clone is expected, so some implementations do one thing, and other implementations the other. For this reason, ICloneable isn't used much, and its further use is actively being discouraged - see the excellent Framework Design Guidelines for more details.

If you dig further into the BCL, you may discover that System.Object has the protected MemberwiseClone method. While you can't call this method directly from another type, you can use this method to implement cloning in your own objects.

A common cloning pattern is to define a protected constructor of the class you want to clone and pass an already existing instance as a parameter. Something like this:

public class MyClass()
{
    public MyClass() {}

    protected MyClass(MyClass other)
    {
        // Cloning code goes here...
    }

    public MyClass Clone()
    {
        return new MyClass(this);
    }
}

However, that obviously only works if you control the type you wish to clone.

If you wish to clone a type that you can't modify, and which doesn't provide a Clone method, you will need to write code to explicitly copy each piece of data from the old instance to the new instance.

Geary

Answer 5

I think the author is asking about copy constructors...

The answer is "yes, but only if you implement it yourself", there's no 'automatic' way of doing it without some heavy cludges (reads: Reflection):

class MyClass {
    private string _name;

    // standard constructor, so that we can make MyClass's without troubles
    public MyClass(string name) {_name = name}
    public MyClass(MyClass old) {
        _name = old._name;
        // etc...
    }
}

The other thing of note in this respect is the IClonable interface, and the .Clone() method this provides. Also .MemberwiseClone() protected method provided by the Object class can help implementing both these methods.

Faso

Answer 6

How about with JSON?

You can get JSON package from : https://www.nuget.org/packages/Newtonsoft.Json/

using Newtonsoft.Json;
public static List<T> CopyAll<T>(this List<T> list) {
  List<T> ret = new List<T>();
  string tmpStr = JsonConvert.SerializeObject(list);
  ret = JsonConvert.DeserializeObject<List<T>>(tmpStr);
  return ret;
}

Poteen

Answer 7

Not any straightforward way that will always work. If your class is [Serializable] or implements ISerializable, you can do a roundtrip serialization to create an identical copy. The same works for [DataContract]

If you only want a shallow copy, you can try Object.MemberwiseClone(). It's a protected method though, and you can only use it from within the class.

If you're lucky, the class implements ICloneable and you can just call the Clone() method.

Subdelirium

Answer 8

An easy way to clone an object is writing it into a stream and read it again:

public object Clone()
{
    object clonedObject = null;
    BinaryFormatter formatter = new BinaryFormatter();
    using (Stream stream = new MemoryStream())
    {
        formatter.Serialize(stream, this);
        stream.Seek(0, SeekOrigin.Begin);
        clonedObject = formatter.Deserialize(stream);
    }

    return clonedObject;
}

But be aware of, that this can cause problems with so called aggregate object.

Hay

Answer 9

Some reasonable solutions here serializing is a valid way to go about it as well as a clone method for each class.. Whipped this up and It seems to work for the few tests i did on it

using System.Reflection; using System.Text.StringBuilder();

public static T CopyClass2<T>(T obj){
    T objcpy = (T)Activator.CreateInstance(typeof(T));
    obj.GetType().GetProperties().ToList()
    .ForEach( p => objcpy.GetType().GetProperty(p.Name).SetValue(objcpy, p.GetValue(obj)));
    return objcpy;

}

And here is the more verbose version I suppose, above suggests you understand Lambda Expressions which isn't common. If you respect readability (Which is totally valid) more I would use below

public static T CopyClass<T>(T obj)
{
    T objcpy = (T)Activator.CreateInstance(typeof(T));
    foreach (var prop in obj.GetType().GetProperties())
    {
        var value = prop.GetValue(obj);
        objcpy.GetType().GetProperty(prop.Name).SetValue(objcpy, value);
    }
    return objcpy;

}

Use this method to list out property's to see if it copied to new reference.. simple it does not list properties of none primitives(structured types).. so you will get a class name

public static string ListAllProperties<T>(T obj)
{
        StringBuilder sb = new System.Text.StringBuilder();
        PropertyInfo[] propInfo = obj.GetType().GetProperties();
        foreach (var prop in propInfo)
        {
            var value = prop.GetValue(obj) ?? "(null)";
            sb.AppendLine(prop.Name + ": " + value.ToString());
        }
        return sb.ToString();
}

Kronstadt

Answer 10

One possibility is to clone it. You have to implement the interface ICloneable and the Clone method.

Roundly

Note that the interface in question has been deprecated. It was never clear whether it meant "deep copy" or "shallow copy"; an interface with an unclear contract is not one that can be accurately implemented or reliably used. (Next time you design a framework base class library, don't repeat this mistake! :) ) – Sholokhov

Answer 11

Do you mean a copy constructor, like it exists in C++ ?

It does not exists in C#. What you can do, is write your own (which is tedious), or you can write a 'Clone' method which uses serialization to create a new instance which has exactly the same values as the original class.

You can serialize the current instance, deserialize it, and return the deserialized result. Disadvantage is that your class has to be Serializable.

Noh

Answer 12

How about something like:

public class MyClass 
{
    int i;
    double d;
    string something;

    public MyClass(int i, double d) {}

    public MyClass Clone()
    {
        return (new MyClass(i, d));
    }
}

Or if you also need to copy something else not usually known when constructing a new object:

    public MyClass CloneMore()
    {
        MyClass obj = new MyClass(i, d);
        obj.something = this.something;
        return (obj);
    }

Call using:

MyClass us = them.Clone();

~~~

Jackinthebox

I think i wrote a class that extends object called memberwiseclone which uses reflection to get/set all the fields/properties. – Lukin

Answer 13

Now a days, it could be a better solution to use Json Serialization and use it as an Extention method like this(Also it does not need to annotate class with [Serializalbe]):

public static class ExtensionMethods
{
    public static T DeepCopyV2<T>(this T source)
    {
        var jsonString = JsonConvert.SerializeObject(source);
        var newObject = JsonConvert.DeserializeObject<T>(jsonString);
    
        return newObject;
    }
}

and use it like:

var newObj = obj.DeepCopyV2();

Mcgrew